The present invention relates to a spark circuit, particularly--but not exclusively--a spark circuit suitable for ignition purposes for heating equipment used on automobiles.
Spark circuits are used to ignite such combustible fuels as gasoline, diesel oil or gas on heating equiqment; they have arrangements which interrupt the flow of electrical current in the primary winding of the ignition coil which causes a high-voltage pulse to be produced in the seconary and the high voltage in turn produces an ignition spark which jumps across a spark gap defined by electrodes which extend into the combustible medium, respectively into the space or chamber of the heating equipment into which such combustible medium is admitted.
Where this type of equipment is used in e.g. private homes, it is mostly operated with a net-current supplied leakage-reactance transformer. For mobile applications, such as motor vehicles, boats, vans or mobile homes, this type of equipment is used, inter alia, to operate heating equipment. If such heating equipment is not operated by the drive of the vehicle itself (e.g. hot water from the radiator), a spark circuit is necessary which receives electrical energy from the vehicle battery or from the generator driven by the vehicle drive.
It is known to use various mechanical chopper or vibrator arrangements which operate according to the principle of Wagner's Hammer; however, these are subject to malfunction, have a relatively short life time and are rather expensive to manufacture, in addition to which they tend to produce high-frequency interference. For this reason it is now most common to use electronic interrupter circuits which are constructed either as self-controlled freely oscillating converters, or else are constructed as remote controlled astable multivibrators. There are various types and special constructions of these electronic circuit arrangements.
In all of them, however, there is a strong dependence of the starting voltage U.sub.Z and the spark energy W.sub.f upon the respective battery voltage or other supply voltage U.sub.bat. As a result, whenever such equipment is used in the conventional vehicle supply systems having 12 volt or 24 volt batteries, respectively generators, especially adapted construction of the spark circuit is needed. In addition, the strong dependency of the characteristic lines for the starting voltage and the spark energy in dependence upon the battery voltage, has the disadvantageous result that particularly in the reduced-voltage operation which is especially important during winter due to cold and weakened batteries, the reduced current supply causes the starting voltage and even more the spark energy to drop. The relationship is primarily proportional to the battery voltage and starting voltage and quadratic as between battery voltage and spark energy. In extreme cases this can mean that too small a starting-ignition voltage and/or too small a spark energy exist to cause ignition, so that the heating system cannot operate. In addition, the known spark circuits have the further disadvantage--and this is particularly true for the remote-controlled spark circuits--that in the overvoltage region there is a high amount of lost-energy heat due to such erasion effects in the exciter winding.